Thank you for most interesting informations! We actually had to put a permeable fabric and stone chippings over it, as we bought a house a year ago with a garden (350m2) with nothing else but a cypress hedge, some yuccas and ficus trees and a grama lawn with millions of dandelions, thistles etc. Almost all the gardens in our corner (rain fall under 300) have stone chippings (usually marble, limestone and some rosa coloured stone), they don't cost much (40 euros a ton) and organic mulch would probably turn into dust. After this was done, I was quite frustrated to find out, how uncomfortable it is to plant through the membrane. There seem to be also different opinions, whether the temperature of the soil under the stones gets warmer or cooler in the summer. It is obviously not possible to improve the soil except around the plants, but our clay would be too hard work for an absentee gardener anyhow. It is a pity, that the plants cannot self seed with this layer, but the weeds have obviously less chance. For me it is a compromise, I am waiting for my new trees and shrubs to make it look greener!
